There's a lot more to it than has been mentioned above
(Jambo, you'll get stiffed for 6K not 10, cos you don't increment on joining, so the first year you're on a par 'ish' - actually less a bit - with what you should be on, so it's the end of 1st, 2nd & 3rd yr increments you don't get, coming inline at end of 4th yr - wouldn't hold your breath for the back-pay either. You'll still get stiffed for about 2K even if you get promoted, cos the increment difference on PAY2000 scale is less than the PAS one).
During the first 4 years, cos you're still on basic plus Flying Pay, you can still lose FP if you have an extended time medically downgraded. Not only that, but if you get medically discharged your ill-health benefit will be lower than it should have been; equally, if you die, your DIS benefit will be lower as well, cos they're both based on your basic.
Here's a clever bit:
Say you've got 2 people at their 22yr point being paid as FS Level 9 and about to join the PAS; one intends to leave after 5 years at around 45 years' old and take the enhanced pension, so he's stayed on AFPS75; the other intends to stay in for full term, so he's transferred to AFPS05.
The first guy will be paid 6K more than the second over the first 4 years as he is getting PAS pay increments, whereas the second guy is standing-still at FS Level 9 until he joins the pay-scale properly at the end of the fourth year.
So the one intending to leave gets more across the period than the one intending to stay - sounds like a clever retention incentive.
Question - if PAS & AFPS05 are going to be tied together in this way, why won't I move to PAS at my 18/40 point ???
Can I have another brick-wall to bang my head against please, this one's getting rather dented 
The NCA/Officer Aircrew desk officers are looking at this - it seems they were caught with their pants down on this too - and should be putting a co-ordinated response to the Pay Policy people (something along the lines of WTF Over 
)
Live from the Swamp 